The present invention relates to signal processing systems. More particularly, the present invention relates to swept spectrum analyzers, and even more particularly to a method and apparatus for a very fast swept spectrum analyzer.
Swept spectrum analyzers have been in use for many years. FIG. 1 depicts one such swept spectrum analyzer system 100. The system 100 includes an RF signal input 10, mixing circuitry 20, intermediate frequency (IF) signal processing circuitry 30, and conversion circuitry 40. Mixing circuitry 20 includes a mixer 21 and a ramping local oscillator (LO) 22. IF processing circuitry 30 includes a resolution bandwidth (RBW)filter 31 and an analog-to-digital converter (ADC) 32.
The operation of system 100 shown in FIG. 1 is as follows. The RF signal input 10 is mixed with the LO using mixer 21 to form an IF signal output. The frequency of the LO signal from oscillator 22 is ramped to cause the IF signal to be swept. The IF signal is then filtered by RBW filter 31. Thus, the swept LO sweeps all the frequencies of the heterodyned-down input signal past the constant frequency of the RBW filter 31, thereby permitting the RBW filter 31 to resolve the input signal""s spectral composition. The filtered IF signal is then converted to a digital signal by ADC 32. The digital representation of the IF signal is then processed by the conversion circuitry 40.
A typical conversion circuitry 40 of the prior art is shown in FIG. 2. Conversion circuitry 40 includes a first mixer 41, a signal generator 42, phase adjustment circuitry 43, a second mixer 44, and a real filter 45. The operation of conversion circuitry 40 shown in FIG. 2 is as follows. The processed IF signal is mixed at mixer 41 with a signal from numeric oscillator 42 to produce an in-phase signal, or I signal. The signal from numeric oscillator 42 is also processed by the phase adjustment circuitry 43 to produce a numeric oscillator signal that is 90xc2x0 out of phase with the signal entering mixer 41. The signal output from the phase adjustment circuitry 43 is then mixed at mixer 44 with the processed IF signal to produce a quadrature signal, or Q signal. The quadrature signal is 90xc2x0 out of phase with the I signal. The I signal and Q signal are then input into and filtered by real filter 45. The filtered I and Q output signals are then processed for display.
Swept signal analysis systems experience a common drawback in that measurement errors result from the sweeping operation. As the sweep speed increases, spectral components of the input signal are swept at increased speeds through the filter. Swept spectrum analyzers traditionally have a sweep time that is proportional to the span, as defined by
xe2x80x83k=TB2/S,
where T is the sweep time, B is the resolution bandwidth, S is the span, and k is a constant. When designing a spectrum analyzer, a designer must choose a default value for k. Traditionally, most applications utilize the constant k set at a value between 1 and 5. If the chosen value for k is too high, the instrument will be too slow. If the chosen value of k is too low, the dynamic range and filter shape display deficiencies.
The deficiencies in the filter output are caused by the LO sweep. Data points at the beginning of the filter range are mixed with a different frequency than data points at the end of the filter range. If the filter length is 1 ms in the time domain, and the sweep ramps 1 kHz in 1 ms, then the beginning point and the end point of the filter would have a 1 kHz difference in the frequencies they represented. If the sweep ramped quickly, the large frequency difference across the beginning and the end of the filter would be considerable. Such a rapid sweep introduces phase errors into the swept filter signal. As a result, the swept filter output loses peak amplitude, widens, and loses shape factor, thereby reducing the utility of the resulting filtered signal.
One approach that has been taken to overcome the sweep rate limitations of traditional swept analysis instruments is to post-process the IF signal to compensate for fast sweeping errors introduced by the LO sweep. One such method is discussed in U.S. Pat. No. 5,117,179 (hereinafter the ""179 patent), which is herein incorporated by reference.
In the ""179 patent, the amplitude loss and frequency shift is dealt with by predicting the loss and applying a compensating sweep-dependent gain or shift. In FIG. 2, the compensating gain is implemented in correction circuitry 46 after the real filter 45. According to the ""179 patent, the compensation for the lost gain may be provided by computing the solution of a swept Guassian response used in the circuit, using a computer model of the actual filter in the circuit, or measuring the actual compensation needed in the spectrum analyzer system. The frequency shift may be compensated in a similar post-processing manner as the lost gain. In such methods, an optimal sweep rate is claimed for Gaussian filters that optimizes the signal to noise ratio for a given sweep time. However, at faster rates the analyzer still exhibits amplitude loss, frequency shifting, and reduced signal to noise ratio characteristics.
The post processing compensation method of correcting swept filters has several disadvantages. The method requires extra hardware to implement the post processing. As faster sweep rates are used, more distortion is produced by the filter and more processing is required to restore the filter output. Even with the extra processing and hardware, it would be very difficult to accurately compensate for both xe2x80x9cnoise-likexe2x80x9d signals and CW signals. Further, as the sweep rate is increased with this method the signal to noise ratio decreases.
What is needed is a swept spectrum analyzer that sweeps frequency arbitrarily fast without creating distortions in the filter output and without decreasing the signal-to-noise ratio.
The present invention solves deficiencies of the prior art by removing a quadratic component of the phase change present in the IF signal. A quadratic component of the phase change is created in the IF signal by mixing a ramping LO signal and an input signal. A quadratic component may be removed by a complex finite impulse response (FIR) filter or other method. In the case of a complex FIR filter, several taps are used to compute the filtered output. Each tap may be multiplied by a phase offset. The phase offset gives each tap a complex value, thereby allowing the filter to remove the quadratic component of the IF signal phase change.